Silicon Cycling in Subglacial Environments

: An Investigation into Glacial Silicon Isotope Systematics

Abstract

Glaciers and Ice Sheets export potentially significant fluxes of dissolved and dissolvable amorphous silica (DSi and ASi), impacting downstream biogeochemical cycles. This could influence our understanding of the wider Si cycle, as glacial Si is not currently considered in global Si models. Enhanced silicate mineral weathering occurs under large ice sheets, due to subglacially stored waters with long residence times. However, subglacial environments are complex, with heterogeneous hydrological systems, and high physical and chemical weathering rates.The silicon isotope (δ³⁰Si) composition of waters and sediments are being increasingly used to understand weathering processes, making them a useful tool for exploring subglacial weathering regimes. This study provides new measurements of δ³⁰Si composition of glacial meltwaters, resulting in the formation of new conceptual models of the subglacial environment. Measurements of DSi and ASi isotopic (δ³⁰DSi and δ³⁰ASi) composition were made for more than twenty Arctic and sub-Arctic glacial rivers, including time-series data for two Greenlandic catchments. This expanded dataset indicates that glacial rivers export significantly light δ³⁰DSi composition compared to non-glacial rivers (+0.16‰ versus +1.38‰). A combination of field and laboratory data suggests that this light δ³⁰DSi composition is driven by physical weathering mechanisms. High subglacial physical erosion rates result in the formation of freshly ground mineral surfaces, enriched in isotopically light Si. The export of isotopically distinct glacial Si could impact understanding of the Si cycle on glacial-interglacial timescales. Previous studies assumed riverine δ³⁰Si composition into the oceans to be relatively stable over time. However, isotopically light glacial Si could impact oceanic δ³⁰Si composition, helping to explain up to 20 - 40% of the oceanic δ³⁰Si composition increase since the last glacial maximum. This demonstrates the importance of constraining the magnitude and drivers of Si fluxes and corresponding δ³⁰Si composition from glacial environments, especially with likely increases in meltwater fluxes under climatic warming scenarios.

Date of Award	25 Jun 2019
Original language	English
Awarding Institution	The University of Bristol
Sponsors	European Research Council
Supervisor	Kate Hendry (Supervisor) & Jemma L Wadham (Supervisor)